source: trunk/doc/src/examples/collidingmice-example.qdoc

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/*!
    \example graphicsview/collidingmice
    \title Colliding Mice Example

    The Colliding Mice example shows how to use the Graphics View
    framework to implement animated items and detect collision between
    items.

    \image collidingmice-example.png

    Graphics View provides the QGraphicsScene class for managing and
    interacting with a large number of custom-made 2D graphical items
    derived from the QGraphicsItem class, and a QGraphicsView widget
    for visualizing the items, with support for zooming and rotation.

    The example consists of an item class and a main function:
    the \c Mouse class represents the individual mice extending
    QGraphicsItem, and the \c main() function provides the main
    application window.

    We will first review the \c Mouse class to see how to animate
    items and detect item collision, and then we will review the \c
    main() function to see how to put the items into a scene and how to
    implement the corresponding view.

    \section1 Mouse Class Definition

    The \c mouse class inherits from QGraphicsItem. The
    QGraphicsItem class is the base class for all graphical items in
    the Graphics View framework, and provides a light-weight
    foundation for writing your own custom items.

    \snippet examples/graphicsview/collidingmice/mouse.h 0

    When writing a custom graphics item, you must implement
    QGraphicsItem's two pure virtual public functions: \l
    {QGraphicsItem::}{boundingRect()}, which returns an estimate of
    the area painted by the item, and \l {QGraphicsItem::}{paint()},
    which implements the actual painting. In addition, we reimplement
    the \l {QGraphicsItem::}{shape()} and \l {QGraphicsItem::}{advance()}.
    We reimplement \l {QGraphicsItem::}{shape()} to return an accurate
    shape of our mouse item; the default implementation simply returns
    the item's bounding rectangle. We reimplement \l {QGraphicsItem::}{advance()}
    to handle the animation so it all happens on one update.

    \section1 Mouse Class Definition

    When constructing a mouse item, we first ensure that all the item's
    private variables are properly initialized:

    \snippet examples/graphicsview/collidingmice/mouse.cpp 0

    To calculate the various components of the mouse's color, we use
    the global qrand() function which is a thread-safe version of the
    standard C++ rand() function.

    Then we call the \l {QGraphicsItem::setRotation()}{setRotation()} function
    inherited from QGraphicsItem. Items live in their own local
    coordinate system. Their coordinates are usually centered around
    (0, 0), and this is also the center for all transformations. By
    calling the item's \l {QGraphicsItem::setRotation()}{setRotation()} function
    we alter the direction in which the mouse will start moving.

        When the QGraphicsScene decides to advance the scene a frame it will call 
        QGraphicsItem::advance() on each of the items. This enables us to animate
        our mouse using our reimplementation of the advance() function.
        
    \snippet examples/graphicsview/collidingmice/mouse.cpp 4
    \snippet examples/graphicsview/collidingmice/mouse.cpp 5
    \snippet examples/graphicsview/collidingmice/mouse.cpp 6

    First, we don't bother doing any advance if the step is 0 since we want to our advance in 
        the actual advance (advance() is called twice, once with step == 0 indicating that items 
        are about to advance and with step == 1 for the actual advance). We also ensure that the 
        mice stays within a circle with a radius of 150 pixels.

    Note the \l {QGraphicsItem::mapFromScene()}{mapFromScene()}
    function provided by QGraphicsItem. This function maps a position
    given in \e scene coordinates, to the item's coordinate system.

    \snippet examples/graphicsview/collidingmice/mouse.cpp 7
    \snippet examples/graphicsview/collidingmice/mouse.cpp 8
    \snippet examples/graphicsview/collidingmice/mouse.cpp 9
    \codeline
    \snippet examples/graphicsview/collidingmice/mouse.cpp 10

    Then we try to avoid colliding with other mice.

    \snippet examples/graphicsview/collidingmice/mouse.cpp 11

    Finally, we calculate the mouse's speed and its eye direction (for
    use when painting the mouse), and set its new position.

    The position of an item describes its origin (local coordinate (0,
    0)) in the parent coordinates. The \l {QGraphicsItem::setPos()}
    function sets the position of the item to the given position in
    the parent's coordinate system. For items with no parent, the
    given position is interpreted as scene coordinates. QGraphicsItem
    also provides a \l {QGraphicsItem::}{mapToParent()} function to
    map a position given in item coordinates, to the parent's
    coordinate system. If the item has no parent, the position will be
    mapped to the scene's coordinate system instead.

    Then it is time to provide an implementation for the pure virtual
    functions inherited from QGraphicsItem. Let's first take a look at
    the \l {QGraphicsItem::}{boundingRect()} function:

    \snippet examples/graphicsview/collidingmice/mouse.cpp 1

    The \l {QGraphicsItem::boundingRect()}{boundingRect()} function
    defines the outer bounds of the item as a rectangle. Note that the
    Graphics View framework uses the bounding rectangle to determine
    whether the item requires redrawing, so all painting must be
    restricted inside this rectangle.

    \snippet examples/graphicsview/collidingmice/mouse.cpp 3

    The Graphics View framework calls the \l
    {QGraphicsItem::paint()}{paint()} function to paint the contents
    of the item; the function paints the item in local coordinates.

    Note the painting of the ears: Whenever a mouse item collides with
    other mice items its ears are filled with red; otherwise they are
    filled with dark yellow. We use the
    QGraphicsScene::collidingItems() function to check if there are
    any colliding mice.  The actual collision detection is handled by
    the Graphics View framework using shape-shape intersection. All we
    have to do is to ensure that the QGraphicsItem::shape() function
    returns an accurate shape for our item:

    \snippet examples/graphicsview/collidingmice/mouse.cpp 2

    Because the complexity of arbitrary shape-shape intersection grows
    with an order of magnitude when the shapes are complex, this
    operation can be noticably time consuming. An alternative approach
    is to reimplement the \l
    {QGraphicsItem::collidesWithItem()}{collidesWithItem()} function
    to provide your own custom item and shape collision algorithm.

    This completes the \c Mouse class implementation, it is now ready
    for use. Let's take a look at the \c main() function to see how to
    implement a scene for the mice and a view for displaying the
    contents of the scene.

    \section1 The Main() Function

    In this example we have chosen to let the \c main() function
    provide the main application window, creating the items and the
    scene, putting the items into the scene and creating a
    corresponding view.

    \snippet examples/graphicsview/collidingmice/main.cpp 0

    First, we create an application object and call the global
    qsrand() function to specify the seed used to generate a new
    random number sequence of pseudo random integers with the
    previously mentioned qrand() function.

    Then it is time to create the scene:

    \snippet examples/graphicsview/collidingmice/main.cpp 1

    The QGraphicsScene class serves as a container for
    QGraphicsItems. It also provides functionality that lets you
    efficiently determine the location of items as well as determining
    which items that are visible within an arbitrary area on the
    scene.

    When creating a scene it is recommended to set the scene's
    rectangle, i.e., the rectangle that defines the extent of the
    scene. It is primarily used by QGraphicsView to determine the
    view's default scrollable area, and by QGraphicsScene to manage
    item indexing. If not explicitly set, the scene's default
    rectangle will be the largest bounding rectangle of all the items
    on the scene since the scene was created (i.e., the rectangle will
    grow when items are added or moved in the scene, but it will never
    shrink).

    \snippet examples/graphicsview/collidingmice/main.cpp 2

    The item index function is used to speed up item discovery. \l
    {QGraphicsScene::NoIndex}{NoIndex} implies that item location is
    of linear complexity, as all items on the scene are
    searched. Adding, moving and removing items, however, is done in
    constant time. This approach is ideal for dynamic scenes, where
    many items are added, moved or removed continuously.  The
    alternative is \l {QGraphicsScene::BspTreeIndex}{BspTreeIndex}
    which makes use of binary search resulting in item location
    algorithms that are of an order closer to logarithmic complexity.

    \snippet examples/graphicsview/collidingmice/main.cpp 3

    Then we add the mice to the scene.

    \snippet examples/graphicsview/collidingmice/main.cpp 4

    To be able to view the scene we must also create a QGraphicsView
    widget. The QGraphicsView class visualizes the contents of a scene
    in a scrollable viewport. We also ensure that the contents is
    rendered using antialiasing, and we create the cheese background
    by setting the view's background brush.

    The image used for the background is stored as a binary file in
    the application's executable using Qt's \l {The Qt Resource
    System}{resource system}. The QPixmap constructor accepts both
    file names that refer to actual files on disk and file names that
    refer to the application's embedded resources.

    \snippet examples/graphicsview/collidingmice/main.cpp 5

    Then we set the cache mode; QGraphicsView can cache pre-rendered
    content in a pixmap, which is then drawn onto the viewport. The
    purpose of such caching is to speed up the total rendering time
    for areas that are slow to render, e.g., texture, gradient and
    alpha blended backgrounds. The \l
    {QGraphicsView::CacheMode}{CacheMode} property holds which parts
    of the view that are cached, and the \l
    {QGraphicsView::CacheBackground}{CacheBackground} flag enables
    caching of the view's background.

    By setting the \l {QGraphicsView::dragMode}{dragMode} property we
    define what should happen when the user clicks on the scene
    background and drags the mouse. The \l
    {QGraphicsView::ScrollHandDrag}{ScrollHandDrag} flag makes the
    cursor change into a pointing hand, and dragging the mouse around
    will scroll the scrollbars.

    \snippet examples/graphicsview/collidingmice/main.cpp 6

    In the end, we set the application window's title and size before
    we enter the main event loop using the QApplication::exec()
    function.

        Finally, we create a QTimer and connect its timeout() signal to the advance()
        slot of the scene. Every time the timer fires, the scene will advance one frame.
        We then tell the timer to fire every 1000/33 millisecond. This will 
        give us a frame rate of 30 frames a second, which is fast enough for most animations. 
        Doing the animation with a single timer connect to advance the scene ensures that all the 
        mice are moved at one point and, more importantly, only one update is sent to the screen 
        after all the mice have moved.
*/